Redesign and enable macOS signing (#4883)

# Description of Changes
Re-enable macOS signing and tweak so it runs successfully through CI.
Also changes the runner to use macOS 15 instead of 13, which was
throwing a deprecation warning in GitHub. Note that the runner doesn't
affect the minimum target, which I've still got set to 10.15 (no idea if
it actually works on 10.15 but let's assume that if it builds it works
until someone can test it)

frontend/src-tauri/tauri.conf.json

@@ -50,6 +50,12 @@
       "deb": {
         "desktopTemplate": "stirling-pdf.desktop"
       }
+    },
+    "macOS": {
+      "minimumSystemVersion": "10.15",
+      "signingIdentity": null,
+      "entitlements": null,
+      "providerShortName": null
     }
   },
   "plugins": {